Among the internal combustion engines known today are ones provided with a valve operating device of a forced-valve-opening/closing type that forcibly drives air intake and exhaust valves by means of cams directly or via rocker arms.
Such a valve operating device of the forced-valve-opening/closing type requires both cams for opening the valves (i.e., valve-opening cams) and cams for closing the valves (i.e., valve-closing cams). In the case where the valves are driven by means of these valve-opening and valve-closing cams directly or via rocker arms, some clearances are provided between the valve-opening and valve-closing cams and the valves in consideration of respective machining or manufacturing accuracy and assembling accuracy, thermal expansion/shrinkage, etc. of the valves, rocker arms, cams and other valve operating component parts.
The above-mentioned clearances can be represented by a valve lift amount difference between a valve lift curve that is indicative of relationship between a rotation angle of the valve-opening cam and a valve lift amount, and a valve lift curve that is indicative of relationship between a rotation angle of the valve-closing cam and a valve lift amount, as will be explained below.
FIG. 13 is a graph showing operating characteristics of the conventionally-known valve-opening and valve-dosing cams, where the vertical axis represents the valve lift amounts, valve speeds determined by one of the valve lift amounts and valve acceleration determined by the valve speed while the horizontal axis represents the cam rotation angles. The valve lift curve 301 of the valve-opening cam, which is a curve having a middle curve section of a high mountain shape, has an inflexion point 302 at a cam rotation angle θ1, inflexion point 303 at a cam rotation angle θ3 and maximum lift point 304 at a cam rotation angle θ2.
The valve lift curve 306 of the valve-closing cam is a curve plotted by displacing the above-mentioned valve lift curve 301 upwardly by a clearance CC, and it has two inflexion points 307 and 308 and maximum lift point 309.
The valve speed curve 311, which is obtained by differentiating one of the above-mentioned valve lift curves 301 and 306, has a maximum speed point 312 corresponding to the inflexion points 302 and 307 of the valve lift curves 301 and 306, a zero speed point 313 corresponding to the maximum lift points 304 and 309 of the curves 301 and 306, and a minimum speed point 314 corresponding to the inflexion points 303 and 308 of the curves 301 and 306.
Although separate valve speed curves are obtained separately in correspondence with the valve lift curves 301 and 306, only one of the valve speed curves 311 is shown and described here because the valve speed curves corresponding to the valve lift curves 301 and 306 are of the same shape.
The above-mentioned maximum speed point 312 is a “jumping point” where the follower (provided directly on the air intake valve or exhaust valve or on the rocker arm) moves or jumps away from (i.e., disengages from) the operating surface (i.e., cam surface) of the valve-opening cam. Further, reference numeral 316 in FIG. 13 represents a landing point where the follower lands on the cam surface of the valve-closing cam. Furthermore, VU represents a valve speed at the maximum speed point 312, and ΔVU represents a difference between the valve speed at the maximum speed point (jumping point) 312 (i.e., jumping speed) and a valve speed at the landing point 316 (i.e., landing speed). The landing speed is a speed at which the follower lands on the cam surface of the valve-closing cam; it should be noted here that the landing speed is distinguished from a colliding speed at which the follower collides against the cam surface of the valve-closing cam (the colliding speed corresponds to the above-mentioned speed difference ΔVU).
Similarly, the above-mentioned minimum speed point 314 is a “jumping point” where the follower moves or jumps away from the cam surface of the valve-closing cam. Further, reference numeral 318 in FIG. 13 represents a landing point where the follower lands on the cam surface of the valve-opening cam. Furthermore, VL represents a valve speed at the minimum speed point 314, and ΔVL represents a difference between the jumping speed at the minimum speed point (jumping point) 314 and a landing speed at the landing point 318. The landing speed is a speed at which the follower lands on the cam surface of the valve-opening cam; it should be noted here that the landing speed is distinguished from a colliding speed at which the follower collides against the cam surface of the valve-opening cam (the colliding speed corresponds to the above-mentioned speed difference ΔVL).
The valve acceleration curve 321, which is obtained by differentiating the above-mentioned valve speed curve 311, has a zero acceleration point 322 corresponding to the maximum speed point 312 of the valve speed curve 311, a minimum acceleration point 323 corresponding to the zero speed point 313 of the valve speed curve 311, and a zero acceleration point 324 corresponding to the minimum speed point 314 of the valve speed curve 311.
Although separate valve acceleration curves are obtained separately from the valve speed curves obtained in correspondence with the valve lift curves 301 and 306 as noted above, only one of the valve acceleration curves 321 is explained because the two valve acceleration curves are of the same shape.
As stated above, the clearance CC is provided between the valve lift curves 301 and 306. Thus, in the case where the valves are driven by the cams directly, the intake valve and exhaust valve first temporarily move away from the valve-opening cam and valve-closing cam and then collide with the cams, because of the provision of the clearance CC between the cams. In the case where the valves are driven by the cams via the rocker arms, on the other hand, the rocker arms first temporarily move away from the valve-opening cam and valve-dosing cam and then collide with the cams, because of the provision of the clearance CC between the cams. Thus, in both of the cases, unwanted sound noise would be produced by the provision of the clearances between the cams.
Particularly, the inflexion point 302 of the valve lift curve 301 is where the operated member (i.e., the air intake valve, exhaust value or rocker arm), slidably contacting the valve-opening cam, moves away from the operating surface of the valve-opening cam, and the inflexion point 308 of the valve lift curve 306 is where the operated member (i.e., the air intake valve, exhaust value or rocker arm), slidably contacting the valve-closing cam, moves away from the operating surface of the valve-closing cam; thus, the valve speeds take maximum absolute values at these inflexion points. Consequently, at these inflexion points, speeds at which the operated members collide with the operating surfaces of the valve-opening and valve-closing cams become great, which would result in increased sound noise.
In order to prevent such unwanted sound noise, there have been proposed, for example in Japanese Patent Application Laid-Open Publication No. SHO-60-108513 (hereinafter referred to as “Patent Literature 1”) or No. HEI-6-221119 (hereinafter referred to as “Patent Literature 2”), an improved valve operating device and cam-profile setting method for an internal combustion engine of the forced-valve-opening/closing type, which are characterized in that the clearance between the valve lift curve of the valve-opening cam and the valve lift curve of the valve-closing cam is partly narrowed.
FIG. 14 is a graph showing relationship between the valve lift amounts and the cam rotation angle in the valve operating device for an internal combustion engine disclosed in Patent Literature 1. In the figure, reference character A represents a cam curve of the valve-opening cam, B represents a cam curve of the valve-dosing cam defining a predetermined clearance with respect to the cam curve A, and D represents a cam curve of the valve-closing cam obtained by modifying the cam curve B so as to define a modified clearance with respect to the cam curve A. Namely, in the cam curve D, a curvature in a region “K” between a maximum lift point PE of the cam curve B and a jump start point PD, at which a slipper of a rocker arm driven by the valve-closing cam jumps away from the cam surface of the valve-closing cam toward the cam surface of the valve-opening cam, is set such that the clearance between the cam curves A and D is greater than the clearance between the cam curves A and B.
More specifically, in the cam curve D, the jump start point PD is located more rearward, in a rotational direction of the cam, than an inflexion point PB of the cam curve B, namely, closer to the maximum lift point PE of the cam curve B, and a point at which the slipper of the rocker arm jumps from the jump start point PD toward the cam curve A is not only located closer to the maximum lift point PE than an inflexion point PA2 of the cam curve A but also set in a first region “L”, as counted from the inflexion point PB, among four equally-divided regions of a range from the inflexion point PB to the maximum lift point PE of the cam curve B. Further, PA1 in FIG. 14 represents a point where the slipper shifts to the cam curve A after jumping away from the cam curve B. Thus, a section where the slipper of the rocker arm shifts from the cam surface of the valve-closing cam (cam curve D) to the cam surface of the valve-opening cam (cam curve A) has a steep incline, so that impact with which the slipper having jumped at the jump start point PD collides against the cam surface of the valve-opening cam (cam curve A) will be reduced considerably.
FIG. 15 is a graph showing relationship between the valve lift amounts and valve train's inertial force and the cam rotation angle in the valve operating device for an internal combustion engine disclosed in Patent Literature 2. In FIG. 15, the vertical axis represents the valve lift amounts and valve train's inertial force, while the horizontal force represents the cam rotation angles.
Further, in FIG. 15, E represents a valve lift curve of the valve-opening cam, F represents a valve lift curve of the valve-closing cam defining a predetermined clearance with respect to the valve lift curve E, G represents a valve lift curve of the valve-closing cam obtained by modifying part of the valve lift curve F, H represents a curve of the valve train's inertial force, C represents a difference between base circle diameters of the valve-opening cam and valve-closing cam.
Between the valve lift curve E and valve lift curve G, there are formed a clearance C0 (e.g., C0=0.25 mm for the air intake valve or C0=0.35 mm for the exhaust valve) in the valve-opening state, clearance C1 (e.g., C1 is about 0.05 mm) at a cam rotation angle J where the direction of the valve train's inertial force changes, and clearance C2 (=C1) at the time of a maximum valve lift.
With the technique shown in FIG. 14 (i.e., disclosed in Patent Literature 1), the clearance between the cam curves D and A in the above-mentioned region “L”, machining or manufacturing accuracy and assembling accuracy decreases as the cam rotation angle increases. If the clearance is small like this, the machining or manufacturing accuracy and assembling accuracy of the component parts of the valve train, such as the valve-opening and valve-closing cams, rocker arms and air intake and exhaust valves, has to be enhanced, which would unavoidably invite cost increase.
With the technique shown in FIG. 15 (i.e., disclosed in Patent Literature 2), the clearance is minimized as close to zero as possible over the range from the maximum lift point to the point of the cam rotation angle J where the direction of the valve train's inertial force changes, and thus, the component parts of the valve train, such as the valve-opening and valve-closing cams, rocker arms, air intake and exhaust valves, must be manufactured and assembled with high accuracy as in the case of the technique disclosed in Patent Literature 1, so that high-accuracy clearance management would require increased necessary cost. Further, if the clearance is small, lubricating oil between the valve-opening and valve-closing cams and the rocker arms would have increased viscosity resistance and agitation resistance, which tends to lower the output and fuel efficiency of the internal combustion engine.